Recent growing seasons have not been typical ones, and this season seems to follow this pattern. Each unusual season has unique disease problems. This year, early planted soybeans are in flowering stage and we are starting to see a different set of issues. There are three things to look for when scouting these fields.

Rusts in soybean and corn The national soybean rust website shows that soybean rust has retreated to the very southern corner of Texas. The disease is so far south that it will not be a concern in the near future. This is because cold winters in the south reduced rust infected kudzu leaves. The disease had no buildup in April and May. Despite this, there was limited movement of this disease last week. The risk will be very low even though there has been plenty of rainfall so far in the Delta Region and north central region.

In contrast, common corn rust should have a good spread this season. By our calculations, spores of the disease should have reached central Missouri around mid-May, southern Iowa no later than the end of May, and central Iowa no later than the first week of June. Experienced scouts should be able to detect the first generation of common rust in central Iowa, which is a low level. In wet southeastern Iowa, the chance to find the disease is high. This season this disease seems to be moving north earlier. Keep in mind that the risk of this disease will be determined by July and August weather.

Brown spotBrown spot is a disease to watch for during the current soybean growth stage. The disease is present in Iowa every year in every field. The fungal pathogen normally infects leaves in the lower portion of the plants. Symptoms of the disease are many irregular, dark brown spots on both upper and lower leaf surfaces. The brown spot pathogen also survives in crop residues and can be seed borne too. In a wet season, it is also dispersed from the soil to soybean plants by splashing rain. In a normal year the disease causes no significant yield losses unless premature defoliation occurs. Frequent rainfall is the primary conditions for an epidemic. In a wet summer, the disease progresses rapidly from lower leaves to upper leaves. Early season buildup increases the risk in fall.

White mold mushroomsWith early planted soybean at R1 growth stage now, some producers may be preparing to spray against white mold infection. To assist in making the decision to spray, one could scout for white mold mushrooms when the canopy is near closure, from late June to early July. White mold mushrooms, also called apothecia, are produced on or near the soil surface. Other mushrooms can occur on the soil surface in soybean fields and be mistaken for white mold mushrooms. The most common is bird’s nest mushroom. Bird’s nest mushrooms are similar in size and shape to the white mold mushrooms. When bird’s nest mushrooms are mature, they darken and have several egg-like “seeds” inside.

Bird's nest mushrooms that can be found in fields right now are easily mistaken for white mold mushrooms.

White mold mushrooms

XB Yang is a professor of plant pathology with responsibility in research and extension. Linus Li is a soybean research associate and SS Navi is assistant scientist working on soybean diseases. Yang can be contacted by email at xbyang@iastate.edu or by phone at (515) 294-8826.

The widespread rain that has occurred across Iowa has been favorable for the development of early season disease in both corn and soybean. In corn, symptoms of anthracnose leaf blight (Figure 1) are common in corn-following corn fields. A very low prevalence of eyespot and common rust have also been seen in ISU fungicide trials. In soybean, brown spot (Figure 2) and bacterial blight (Figure 3) symptoms are becoming common.

Rain favors the dispersal of fungal pathogens that survive in infested crop residue. The anthracnose leaf blight pathogen produces spores in a jelly-like substance on the surface of infested crop residue. Rain splashes the spores onto the lower leaves on corn plants where infection occurs. The fungus survives in infested surface crop residue for approximately 10 months. Thus, anthracnose leaf blight is usually very common in no- or reduced-tillage corn-following-corn (CC) fields, but it is rarely seen in corn-following-soybean fields (CSB). In 2008, in a rotation field trial in southeast Iowa, the mean incidence of anthracnose leaf blight in CC plots was 97.3 percent, while in CSB, the incidence was 0 percent. In this same trial, mean anthracnose stalk rot severity was 1.65 and 1.85 (using the U. Illinois 0-5 system where 0 is healthy and 5 is lodged) in CC and CSB plots, respectively. Thus, there was a poor relationship between anthracnose leaf blight and anthracnose stalk rot.

Figure 1. Symptoms of anthracnose leaf blight

Brown spot occurs on the bottom most leaves of soybean plants. Severity of the disease increases with increasing periods of leaf wetness. Brown spot lesions are small, irregular-shaped and dark brown. Adjacent lesions often grow together and form larger blotches. Infected leaves quickly turn “banana” yellow and drop.

Figure 2. Brown spot symptoms

Bacterial blight occurs on the upper leaves of the canopy. Rain favors infection of bacterial pathogens because bacteria do not produce the necessary enzymes to directly infect plant tissue, thus they rely on wounding (bruising, tearing) caused by rain and wind, to allow them entry to the host plant and enable infection to occur. Bacterial blight lesions are angular with reddish-brown centers and water-soaked margins surrounded by “lemon” yellow halos. Lesions often grow together to produce large, irregularly shaped dead areas, which fall out, causing the leaves to appear tattered.

Figure 3. Symptoms of bacterial blight

From a management perspective, all of these diseases cause little if any yield loss. There are no chemicals available that can be used to control bacterial diseases. Although, most registered fungicides are labeled for anthracnose leaf blight and brown spot, the lower leaves of the crop that are impacted by these diseases contribute little to yield. Furthermore, the rapidly growing corn plan will outgrow anthracnose leaf blight infection within the next week or so. In very wet years, brown spot may develop into the mid canopy of soybean plants and a fungicide application at around R3 may protect yield.

Alison Robertson is an assistant professor of plant pathology with research and extension responsibilities in field crop diseases. Robertson may be reached at (515) 294-6708 or by email at alisonr@iastate.edu.

A combination of low glyphosate prices deterring preemergence applications and a wet June preventing timely postemergence applications has resulted in weeds reaching levels capable of interfering with crop growth and yield potential in fields across the state. While many fields will still be able to be ‘cleaned up’ when they dry, the lost yield cannot be recovered.

While the lost yield is unfortunate, it can provide a teachable moment regarding crop-yield interference. WeedSOFT 8X is a web-based competition model that can be used to estimate early-season yield losses. All that is required to determine potential yield loss from weed competition is crop stage, estimated weed-free yield potential, weed density and weed size. The model is based on research conducted in the 1990s at land-grant universities across the Corn Belt.

To demonstrate WeedSOFT, weed infestations in two soybean fields outside of Ames were sampled on June 15 by counting weeds in 10 one-square-foot quadrats. The soybean, planted in 30 inch rows, were at the V4 growth stage and had an estimated yield potential of 65 Bu/A. The yield already lost at the time of sampling and if control is delayed until the V5 stage as predicted by WeedSOFT is presented in the following table.

Field 1 had a heavy infestation of giant foxtail, and WeedSOFT estimated that the foxtail and other weeds had already reduced yields by 6.7 bu/A (10 percent). The second field had fewer total weeds, but the weed species present were more competitive than foxtail and also were significantly larger than the weeds in Field 1, resulting in a loss estimate of 7.4 bu/A. It is important to recognize that the specific yield loss relationship between weeds and crops is highly variable depending upon the environment and cultural practices. The estimates made by WeedSOFT are conservative, thus the model is more likely to overestimate than underestimate the yield loss. However, the estimates are based on the best research available and are realistic.

The use of WeedSOFT can demonstrate the risks associated with total postemergence programs and the value of early-season weed control. The focus for this year should be to select a postemergence program that will effectively control the weeds present in individual fields. For next year, adjust plans accordingly to minimize the risk of losing yield potential to early-season competition.

Bob Hartzler is a professor of agronomy with extension, teaching and research responsibilities.

In this week's crop minute, two ISU Extension specialists discuss issues related to the overly wet growing season - soil nitrogen needs and crop diseases popping up with the rain.

John Sawyer, ISU Extension soil fertility specialist, addresses concerns related to the impact of wet soils conditions on nitrogen management and nitrogen remaining in the soil for crop. He talks about estimating nitrogen loss and applying additional nitrogen.

Alison Robertson, ISU Extension field crops pathologist, says that all the rain we are having favors the development of bacterial diseases and dispersion of pathogens. She talks about diseases that are popping up this time of year.

Many areas of central to southern Iowa have experienced well above normal rainfall this spring, with several large rainfall events (see map). In the early spring, cold soils help reduce potential for nitrate-N loss due to slow accumulation of nitrate and slow denitrification. However, continued wet soil conditions into June with warm soils, prolonged saturation and tile drainage enhance nitrate loss.

One way to determine nitrogen (N) loss is to calculate an estimate. Predicting the exact amount is quite difficult as many factors affect losses. However, estimates can provide guidance for supplemental N applications.

Research Measurement of Nitrate LossResearch conducted in Illinois* indicated approximately four to five percent loss of nitrate-N by denitrification per day that soils were saturated. An all-nitrate fertilizer was applied when corn was in the V1 to V3 growth stage (late May to early June). Soils were brought to field capacity and then an excess four inches of water (above ambient rainfall) was applied by irrigation evenly over a three-day period (which maintained saturated soils for three to four days on the finer textured soils) or an excess of six inches of water was applied over an eight-day period (which saturated soils an additional three to four days).

The excess water application resulted in loss of 60 to 70 pounds N/acre on silt loam and clay loam soils, due to denitrification loss. On a very coarse-textured, sandy soil, virtually all nitrate-N was moved out of the root zone by leaching. On the finer textured soils, an addition of 50 pounds N/acre after the excess water was sufficient to increase corn yields to approximately the same level where no excess water was applied. This was not the case on the sandy soil because considerably more N was lost due to leaching.

Nitrate loss via tile drainage does increase with above normal rainfall. At the Gilmore City, Iowa ag-drainage research site where tile-flow nitrate has been monitored since 1990, nitrate loss is greatest in years with higher precipitation and hence greater tile flow. At N fertilization rates of 150 to 160 pounds N/acre, the annual nitrate-N loss per acre was 52 pounds in the 1990-1993 period, 9 pounds in the 1994-1999 period, and 39 pounds in the 2000-2004 period (average nitrate-N losses for the combined corn-soybean sequence). The range in yearly nitrate-N loss for the years studied was 1.0 pound nitrate-N/acre in 1997 to 75 pound nitrate-N/acre in 1990.

Typically a high portion of tile flow and associated nitrate-N loss occurs in the springtime. The impact of excess precipitation on potential for nitrate remaining in the soil for crop use in wet springs like this year is that more nitrate-N is lost via tile flow, and overall the annual loss would be in the range of perhaps twice the “normal” loss amount, increasing from around 15-25 pounds N/acre to 40-50 pounds N/acre.

Nitrate LossAccording to research at the University of Nebraska, the estimated denitrification loss of nitrate when the soil temperature is 55 to 60 degrees F is 10 percent when soil is saturated for five days and 25 percent when saturated for 10 days (2 to 2.5 percent per day). Loss increases with warmer soils. Illinois research conducted in late May to early June (soil temperatures greater than 65 degrees F) with excess application of water on silt loam and clay loam soils indicated approximately four to five percent loss of nitrate present per day that soils were saturated.

To estimate N loss, the first step is to estimate the amount of ammonium converted to nitrate-N. By now, one might assume late fall anhydrous ammonia and manure ammonium to be nearly converted to nitrate, and with early April preplant N applications, much more than 50 percent converted to nitrate. Less conversion to nitrate would occur with use of a nitrification inhibitor. Recent ammonium applications (within the last two weeks) would still be predominantly in the ammonium form, especially for anhydrous ammonia. Recent application of nitrate-containing fertilizers would result in more nitrate being present. Urea-ammonium nitrate solutions (28 or 32 percent UAN) contain one-quarter nitrate-N, and nitrify more rapidly. The second step is to estimate the percentage of nitrate-N loss as described in the research above. The amount of N loss is calculated from these two estimates.

ExampleThe following might be an example of a situation with a spring preplant application of UAN solution and the wet conditions encountered this year. If 85 percent of a 120 pound N application is converted to nitrate, and soils were then saturated for 10 days when warm, the N loss estimate would be (120 pounds N per acre x 85 percent nitrate/100) x (4 precent per day/100) x (10 days) = 40 pounds N per acre. Add in increased tile flow on tile-drained fields, and the loss estimate could be 60 pounds N per acre. Variation of lower or higher losses could easily occur depending on warmer or cooler conditions, different forms of applied N, more or less time from N application to wet conditions and more or less time and frequency soils are saturated. The same will occur for different landscape positions and soils. With very coarse-textured/sandy soils, significant rainfall events (four to six inches or more) in addition to already moist soils could easily result in all nitrate leaching out of the crop rooting zone.

*Reported in the 1993 Iowa State University Integrated Crop Management Conference proceedings, pp. 75–89, and in Torbert et al., 1993, “Short-term excess water impact on corn yield and nitrogen recovery,” Journal of Production Agriculture 6:337–344.

John Sawyer is a professor of agronomy with research and extension responsibilities in soil fertility and nutrient management.

When conventional application equipment can be moved through the field (i.e., the soils are dry enough and the corn is short enough), then injection of anhydrous ammonia or UAN solutions would top the list of best options. Next would come urea-ammonium nitrate solution (UAN) surface dribbled between corn rows, and then broadcast urea. Broadcast UAN solution should be avoided on corn larger than the V7 growth stage. With tall corn, supplemental UAN will need to be applied with high-clearance equipment. Injection coulters or drop tubes between every other row or every row should work equally well. Urea can be broadcast with buggy or high clearance dry box spreaders if they can be driven between corn rows or aerially applied. For broadcast urea, use of a urease inhibitor can help slow volatile N loss from warm wet soils as they dry. A urease inhibitor would not be needed with injected UAN, and low probability of need with surface dribbled UAN due to limited UAN surface contact with soil. With broadcast urea, some material will fall into the plant whorls, but will cause only cosmetic damage to leaf tissue that will show as spots or streaks when the leaf grows out of the whorl. Of course to get benefit from surface applied N it needs to be moved into the root zone with rainfall.

John Sawyer is a professor of agronomy with research and extension responsibilities in soil fertility and nutrient management.

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